Heat shrinkable polyester film

ABSTRACT

The present invention provides a heat shrinkable polyester film whose hot water shrinkage along its main shrinkage direction is equal to or greater than 20% when the film is put in hot water of 70° C. for 5 sec; 35% to 55% when put in hot water of 75° C. for 5 sec; and 50% to 60% when put in hot water of 80° C. for 5 sec, wherein the compressive strength of the label formed by the film satisfies a following expression (1):
 
Y&gt;X 2.2   (1)
         where y denotes compressive strength (mN) and x denotes film thickness (μm).

BACKGROUND OF THE INVENTION

This application is a reissue of U.S. Pat. No. 6,451,445.

1. Field of the Invention

The present invention relates to a heat shrinkable polyester film, andparticularly to a heat shrinkable polyester film suitable for use in alabel. More particularly, the present invention relates to a heatshrinkable polyester film for use in a label with high compressivestrength, which has very few creases, uniform shrinkage, and minimaldistortion occurring from a heat shrinkage process.

2. Description of the Related Art

Heat shrinkable polyester films made of polyvinyl chloride, polystyrene,or the like, are widely used in labels wrapped around the body ofpolyethylene terephthalate (PET) bottles. However, polyvinyl chlorideproducts pose a hazard as they generate a chlorine gas when incinerated,and polystyrene products cause problems such as printing difficulties.Moreover, in order to recycle PET bottles, it is necessary to separatenon-PET labels thereon which are made of polyvinyl chloride,polystyrene, etc., from the PET bottles. Thus, heat shrinkable filmsmade of polyester, which are free of the above-mentioned problems, havebeen attracting public attention.

However, heat shrinkable polyester films have not been effective asshrinkable films for use in labels, because many of them shrink toorapidly resulting in creases, non-uniform shrinkage and distortion afterthe shrinkage. Moreover, such films are easily torn by an externallyapplied force after the shrinkage.

In order to avoid some of these problems, Japanese Patent Publicationfor Opposition No. 7-77757 discloses a method for improving a finalshrinkage property by remarkably reducing a tearage strength along adirection perpendicular to its main shrinkage direction.

Moreover, Japanese Laid-Open Publication No. 58-64958 discloses a methodfor improving a final shrinkage property by reducing an orientationreturning stress.

However, the films obtained by the above-mentioned methods could notachieve sufficient final shrinkage properties for use with small PETbottles whose shrink tunnel passing time is short. Thus the films wereineffective as shrinkage films. That is, when placing a tubular shapelabel, which is formed by a shrinkable film, around a PET bottle andperforming a heat treatment to shrink the film onto the PET bottle,crease, non-uniform shrinkage, and distortion have sometimes occurredfrom the film shrinkage process.

In addition, the process speed for beverage bottling lines for PETbottles, and the like, is increasing. Therefore, the label is requiredto have a high-speed application property as well as a good finalshrinkage property as mentioned above. In short, as illustrated in FIGS.1 and 2, when a label 2 is placed around a PET bottle 1 in a high-speedprocess by a pressure member 3, if the high-speed application propertyis poor, the label cannot be applied properly due to insufficientrigidity of the label. The application property of the label is likelyto depend greatly on the rigidity of the film, whereby it is possible toaddress such as problem by increasing a film thickness. However,problems occur when the film thickness increases. For example, a filmthickness increase leads to an increase in weight, which results in apoor handling property. There are also other problems such as the filmthickness increase leading to higher costs.

SUMMARY OF THE INVENTION

According to one aspect of this invention, there is provided a heatshrinkable polyester film whose hot water shrinkage along its mainshrinkage direction is equal to or greater than 20% when the film is putin hot water of 70° C. for 5 sec; 35% to 55% when put in hot water of75° C. for 5 sec; and 50% to 60% when put in hot water of 80° C. for 5sec, wherein the compressive strength of the label formed by the filmsatisfies a following expression (1):y>x^(2.2)  (1)

-   -   where y denotes compressive strength (mN) and x denotes film        thickness (μm).

In one embodiment of the invention, the film has a thicknessdistribution of 6% or less.

According to another aspect of this invention, there is provided a labelfor a bottle which is produced by a heat shrinkable polyester film ofthe present invention.

Thus, the invention described herein makes possible the advantages ofproviding a heat shrinkable polyester film for use in variousapplications including a small PET bottle, with a good high-speedapplication property and a good final shrinkage property, withoutexceeding the film thickness of a conventional film.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a step of placing a shrinkable film around a PETbottle; and

FIG. 2 is a front view illustrating a part of a PET bottle after ashrinkable film is shrunk onto the PET bottle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described.

A heat shrinkable polyester film of the present invention is producedfrom a polyester comprising a dicarboxylic acid component and a diolcomponent.

The dicarboxylic acid component of the polyester contains aromaticdicarboxylic acids (e.g., terephthalic acid, isophthalic acid,naphthalenedicarboxylic acid, and orthphthalic acid), aliphaticdicarboxylic acids (e.g., adipic acid, azelaic acid, sebacic acid, anddecanedicarboxylic acid), and alicyclic dicarboxylic acids, or the like.

Where the polyester contains an aliphatic dicarboxylic acid(s) (e.g.,adipic acid, sebacic acid, and decanedicarboxylic acid, or the like),the content of the aliphatic dicarboxylic acid is preferably less than 3mol % (this applies to all the dicarboxylic acid components used). Aheat shrinkable polyester film obtained by using a polyester whichcontains 3 mol % or more of such an aliphatic dicarboxylic acid(s) islikely to reduce its elongation at rupture along a directionperpendicular to its main shrinkage direction after a shrinkage process,and may not have a sufficient rigidity in a high-speed applicationprocess.

The polyester preferably does not contain a polyvalent carboxylic acidwhose valence is 3 or more (e. g., a trimellitic acid, a pyromelliticacid, and anhydride thereof). If the polyester contains such apolyvalent carboxylic acid, the content thereof is preferably 3 mol % orless. With a heat shrinkable polyester film obtained by using thepolyester containing such a polyvalent carboxylic acid, an elongation atrupture along a direction perpendicular to its main shrinkage directionafter a shrinkage process is likely to be reduced, and a sufficientshrinkage may not be achieved.

The diol component of the polyester to be used in the present inventioncontains aliphatic diols (e.g., ethylene glycol, propane diol, butanediol, neopentyl glycol, hexane diol, or the like), alicyclic diols(e.g., 1,4-cyclohexane dimethanol), and aromatic diols, or the like.

The polyester used in the heat shrinkable polyester film of the presentinvention is preferably a polyester whose glass-transition temperature(Tg) is adjusted to 60° C. to 75° C. by mixing therein one or more diolswhose carbon number is 3 to 6 (e.g., propane diol, butane diol,neopentyl glycol, and hexane diol).

In order to obtain a heat shrinkable polyester film having aparticularly desirable final shrinkage property, neopentyl glycol can beused as one of the diol components, preferably in an amount of 15 mol %to 25 mol % (this applies to all the diol components used).

It is not preferable to contain a diol whose carbon number is 8 or more(e.g., octanediol) or a polyalcohol whose valence is 3 or more (e.g.,trimethylolpropane, trimethylolethane, glycerin, and diglycerin). Ifsuch a diol or such a polyalcohol is contained, the content thereof ispreferably 3 mol % or less. With a heat shrinkable polyester filmobtained by using a polyester containing more than 3 mol % of a diol ora polyalcohol, a sufficient shrinkage may not be achieved.

The polyester preferably contains the least amount possible ofdiethylene glycol, triethylene glycol, and polyethylene glycol.Although, diethylene glycol is likely to be present in the polyesterused in the present invention because it is a byproduct from a polyesterpolymerization process, the content of diethylene glycol is preferablyless than 4 mol %.

A preferable acid component and a diol component used for the polyesterof the present invention are as follows. The main acid component is aterephthalic acid, mixed with 14 mol % or more of an isophthalic acidand an adipic acid less than 5 mol %. Moreover, the main diol componentis an ethylene glycol, mixed with 5 mol % to 15 mol % of butanediol.

When two or more polyesters are mixed together, the above-mentioned acidcomponent content and the diol component content are determined withrespect to the acid component and the diol component, respectively, ofall the polyesters. These contents are irrespective of whether esterinterchange occurs after mixing.

Moreover, in order to improve smoothness of a heat shrinkable polyesterfilm, the film preferably contains an inorganic lubricant and an organiclubricant. The inorganic lubricant may be, for example, titaniumdioxide, fine particle silica, kaoline, or calcium carbonate. Theorganic lubricant may be, for example, a long-chain fatty acid ester.Optionally, the film further contains additives such as a stabilizer, acoloring agent, an antioxidant, a defoamer, an antistatic agent, and anultraviolet sorbent.

The above-mentioned polyesters may be produced through polymerization bya conventional method. For example, the polyester can be obtained byusing, for example, a direct esterification method, where a dicarboxylicacid and a diol are directly reacted with each other, or atransesterification method, where a dimethyl dicarboxylate and a diolare reacted with each other. The polymerization may be performed eitherin a batch process or a continuous process.

The hot water shrinkage of the heat shrinkable polyester film of thepresent invention can be determined as follows. The film is subjected toa no-load treatment in hot water. Based on the length of the film beforeshrinkage and the length after shrinkage, the hot water shrinkage of thefilm can be calculated by the expression: Heat shrinkage=((length beforeshrinkage−length after shrinkage)/length before shrinkage)×100(%). Theheat shrinkable polyester film of the present invention has a hot watershrinkage of 20% or more, and preferably 22% to 35%, along its mainshrinkage direction when the film is put in hot water of 70° C. for 5sec; 35% to 55%, and preferably 40% to 50%, when put in hot water of 75°C. for 5 sec; 50% to 60%, when put in hot water of 80° C. for 5 sec.

When the hot water shrinkage along the main shrinkage direction is lessthan 20% at 70° C. for 5 sec, the low temperature shrinkage of the filmmay be insufficient, whereby it may be necessary to increase theshrinkage temperature, which is undesirable. When the shrinkage exceeds50%, jumping of the label may occur from the heat shrinkage process,which is undesirable.

The hot water shrinkage of the film of the present invention is 35% to55% when put in hot water of 75° C. for 5 sec. When the shrinkage isless than 35%, the film will not sufficiently shrink near the entrancesection of the bottle, which is undesirable (that is, when a bottle orthe like is packaged with the film and passed through a shrink tunnel,the film is likely to have creases and/or non-uniform shrinkage in apetal-like pattern along the edge of the film). When the shrinkageexceeds 55%, the label may still have a shrinking force even after theheat shrinkage process, whereby the label is likely to jump from thebottle.

The hot water shrinkage is 50% to 60% when put in hot water of 80° C.for 5 sec. When the shrinkage is less than 50%, the film will notsufficiently shrink near the entrance section of the bottle, which isundesirable. When the shrinkage exceeds 60%, the label may still have ashrinking force even after the heat shrinkage process, whereby the labelis likely to jump from the bottle.

The compressive strength of a label produced from a heat shrinkablepolyester film of the present invention has a compressive strength whichsatisfies the following expression (1):y>x^(2.2)  (1)

-   -   where y denotes compressive strength (mN) and x denotes film        thickness (μm).

The preferable compressive strength y of a label is as follows.y>1.1x^(2.2)  (1′)

While the compressive strength is influenced by the film thickness,there may be a label application defect due to a high-speed applicationmachine property when the above-mentioned expression is not satisfied.

The thickness x of the heat shrinkable polyester film of the presentinvention is not limited to any particular value. As a heat shrinkablefilm for use in a label, the thickness of the film is preferably 10 μmto 200 μm, and more preferably 20 μm to 100 μm.

Next, a specific example of the method for producing a heat shrinkablepolyester film of the present invention will be described, though thepresent invention is not limited to such a method.

According to the present invention, a polyester material is first driedby using a dryer such as a hopper dryer or a paddle dryer or a vacuumdryer. Then, the polyester material is melted and extruded into a filmat a temperature of 200° C. to 300° C., and rapidly cooled to obtain anundrawn film. The extrusion process may be done with any known methodsuch as a T-die method, or a tubular method.

The obtained undrawn film is then drawn at a temperature equal to orgreater than Tg−5° C. and less than Tg+15° C. (for example, 70° C.−90°C.) along the longitudinal direction (extrusion direction) by a drawingfactor of 1.05 or more, and preferably 1.05 to 1.20. Then, after apre-heating treatment is performed, the film is drawn along thetransverse direction (direction perpendicular to the extrusiondirection) by a drawing factor of 4.5 or more, and preferably 4.7 to 5.2(first drawing step). By pre-heating the film, shrinkage is restrictedand the film rigidity increased. Then, the film is drawn further at atemperature of 65° C. to 85° C. along the transverse direction by adrawing factor of 1.05 or more (second drawing step). Thus, the heatshrinkable polyester film of the present invention is obtained.

By drawing a film in such a two-step drawing process, the film rigidityincreases, and problems can be reduced during a high-speed shrinkage anda high-speed application.

The drawing process may be a biaxial drawing in which the film is drawnin the longitudinal direction and then drawn with a tenter in thetransverse direction. Such a biaxial drawing may be either a successivebiaxial drawing method or a simultaneous biaxial drawing method. Afterthis drawing process, the film may optionally be further drawn in thelongitudinal or transverse direction.

In order to achieve the objects of the present invention, it is morepractical to have the main shrinkage direction along the transversedirection. Therefore, an exemplary film production method where the mainshrinkage direction is along the transverse direction has been describedabove. However, it is understood that the main shrinkage direction mayalternatively be along the longitudinal direction, in which case theabove-described method can be used only with the drawing directionsbeing rotated by 90 degrees.

In the present invention, an undrawn film obtained from a polyester ispreferably drawn at a temperature equal to or greater than Tg−5° C. andless than Tg+15° C.

It is not preferable to draw the film at a temperature less than Tg−5+C.In such a case, the sufficient heat shrinkage, a feature of the presentinvention, may not be obtained, and further the transparency of theobtained film may deteriorate.

In addition, it is not preferable to draw the film at a temperature ofTg+15° C. or more. In such a case, the film may not have sufficientrigidity in the high-speed processes, and further the film may have asubstantially increased thickness variation.

The heat shrinkable polyester film of the present invention preferablyhas a film thickness distribution of 6% or less, and more preferably 5%or less. The film thickness distribution can be calculated based on thefollowing expression: Thickness distribution=((maximum thickness−minimumthickness)/average thickness)×100 (%).

A three-color printing test is performed to evaluate the final shrinkageproperties of the film. With a film having a thickness distribution of6% or less, it is easy to superimpose the colors on one another.However, with a film having a thickness distribution greater than 6%,the colors may not be desirably superimposed on one another.

In order to reduce the thickness distribution of a heat shrinkablepolyester film, it is preferred to heat the film to a predeterminedtemperature with hot air at a low flow rate such that the heat transfercoefficient is 0.0013 cal/cm²·sec·° C. or less in a pre-heat process,which is performed before the drawing process for drawing the film witha tenter in the transverse direction.

In order to suppress heat generation inside the film associated with thedrawing process and thus to reduce the non-uniform film temperature inthe width direction, the heat transfer coefficient in the drawingprocess should be 0.0009 cal/cm²·sec·° C. or more, and preferably 0.0011to 0.0017 cal/cm²·sec·° C.

When the heat transfer coefficient in the pre-heat process exceeds0.0013 cal/cm²·sec·° C. or when that in the drawing process is less than0.0009 cal/cm²·sec·° C., the thickness is likely to be non-uniform. Insuch a case, when a multi-color printing is performed on the obtainedfilm, the colors of a printed pattern may not be properly superimposedon one another.

A label of the present invention can be produced as follows. First, asheet of heat shrinkable film in a rectangular shape having apredetermined size is rolled into a tubular shape with two of itsopposite edges bonded together. Then, the tubular film is cut intosmaller label pieces. The method of bonding is not limited to anyparticular method. For example, a solvent or a swelling agent is appliedon at least one of two opposite edges of the heat shrinkable polyesterfilm, the opposite edges are bonded together before the solvent or theswelling agent dries up.

EXAMPLES

The present invention will now be described in greater detail by way ofexamples. However, the present invention is not limited to theseexamples, but other examples may be possible without departing from thescope of the present invention.

The evaluation methods used in the present invention will now bedescribed.

(1) Heat Shrinkage

A sheet of film was cut into 10 cm×10 cm pieces, and subjected to ano-load treatment in hot water for a predetermined time at apredetermined temperature of±0.5° C. so as to heat-shrink the film.Then, the longitudinal and transverse dimensions of the film weremeasured so as to obtain the heat shrinkage of the film according to thefollowing expression (2). The direction along which the heat shrinkagewas greater was assumed to be the main shrinkage direction of the film.Heat shrinkage={(length before shrinkage−length after shrinkage)/lengthbefore shrinkage}×100(%)  (2)(2) Final Shrinkage Property

A three-color printing test using three colors of yellow, gold and whitemanufactured by Toyo Ink Mfg. Co., Ltd. was performed on a heatshrinkable film in advance.

A test was conducted using in each a round bottle sample of 500 ml (20.6cm in height, 6.5 cm in diameter at the center, a bottle of “Gogo noKocha” of Kirin Beverage Corporation manufactured by Yoshino Kogyo Co.,Ltd.), which was passed through a Steam Tunnel (Model: SH-1500-L)manufactured by Fuji Astec Inc for a passage time of 2.5 sec, zonetemperature of 80° C. (20 samples were evaluated).

The evaluation was done by visual observation with the criteria shownbelow.

-   -   ∘: No creasing, jumping or insufficient shrinkage observed    -   X: Creasing, jumping or insufficient shrinkage observed        (3) Compressive Strength

A square tubular label a 120 mm in height and 175 mm in diameter widthwhen folded flatly into two was produced to measure its compressivestrength in a vertical direction. A strograph (Model: V10-C)manufactured by Toyo Seiki Co., Ltd. was used to measure a maximum valueof compressive strength, (mN) at a cross head speed of 200 mm/min in acompressive mode (5 samples were evaluated).

(4) Tg (Glass-transition Temperature)

A differential scanning calorimeter (DSC) (Model: DSC220) manufacturedby Seiko Instruments & Electronics Ltd. was used to raise thetemperature of an undrawn film (10 mg) from −40° C. to 120° C. at atemperature rising rate of 20° C./min to obtain an endothermic curve.The glass-transition temperature of the film was obtained based on theobtained endothermic curve. A tangential line was drawn immediatelybefore and after an inflection point along the endothermic curve toobtain Tg (glass-transition temperature) at the intersection between thetangential lines.

(5) Thickness Distribution

A contact thickness meter (Model: KG60/A) manufactured by AnritsuCorporation was used to test the thickness of the sample which was 5 cmalong the longitudinal direction and 50 cm along the transversedirection (20 samples were evaluated). The thickness distribution(thickness variation) of each sample was tested by the expression (3) asshown below. In addition, the average value (n=50) of the thicknessdistribution was evaluated by the criteria shown below.Thickness distribution=((maximum thickness−minimum thickness)/averagethickness)×100(%)  (3)

-   -   ∘: equal to or less than 6%    -   Δ: greater than 6% and less than 10%    -   X: equal to or more than 10%

The following polyesters were used in the examples and comparativeexamples to be described below.

-   -   Polyester A: polyethylene terephthalate (intrinsic viscosity        (IV): 0.75 dl/g)    -   Polyester B: a polyester comprising 78 mol % of terephthalic        acid, 22 mol % of isophthalic acid and ethylene glycol (IV: 0.72        dl/g)    -   Polyester C: a polyester comprising 65 mol % of terephthalic        acid, 10 mol % of isophthalic acid, 25 mol % of adipic acid and        butanediol (IV: 0.77 dl/g)    -   Polyester D: a polyester comprising 70 mol % of terephthalic        acid, 30 mol % of adipic acid and ethylene glycol (IV: 0.70        dl/g)    -   Polyester E: a polyester comprising 53 mol % of terephthalic        acid, 47 mol % of sebacic acid and ethylene glycol (IV: 1.10        dl/g)    -   Polyester F: a polyester comprising terephthalic acid, 70 mol %        of ethylene glycol and 30 mol % of neo pentyl glycol (IV: 0.72        dl/g)    -   Polyester G: polybutylene terephthalate (IV: 1.20 dl/g)

Example 1

A polyester obtained by mixing together 20 wt % of Polyester A, 70 wt %of Polyester B and 10 wt % of Polyester C was melted and extrudedthrough a T die at a temperature of 280° C., and rapidly cooled down byusing a chill roll to obtain an undrawn film. The Tg of the undrawn filmwas 69° C.

The undrawn film was drawn in the longitudinal direction by a drawingfactor of 1.1 at a film temperature of 80° C. After the film waspre-heated until reaching 88° C. under a heat transfer coefficient of0.033 W/(m·° C.), the undrawn film was drawn with a tenter in thetransverse direction at a film temperature of 70° C. by a drawing factorof 4.6 (first drawing step). The film was then subjected to a heattreatment at 70° C. for 10 sec, and further drawn in the transversedirection at the film. temperature of 68° C. by a drawing factor of 1.1(second drawing step) to obtain a heat shrinkable polyester film havinga thickness of 50 μm.

Example 2

A polyester obtained by mixing together 10 wt % of Polyester A, 70 wt %of Polyester B and 20 wt % of polyester D was melted and extrudedthrough a T die at a temperature of 280° C., and rapidly cooled down byusing a chill roll to obtain an undrawn film. The Tg of the undrawn filmwas 67° C.

The undrawn film was drawn in the longitudinal direction by a drawingfactor of 1.1 at a film temperature of 80° C. After the film waspre-heated until reaching 68° C. under the heat transfer coefficient of0.033 W/(m·° C.), the undrawn film was drawn with a tenter in thetransverse direction at the film temperature of 68° C. by a drawingfactor of 4.6 (first drawing step). The film was then subjected to aheat treatment at 69° C. for 10 sec, and further drawn in the transversedirection at the film temperature of 68° C. by a drawing factor of 1.1(second drawing step) to obtain a heat shrink able polyester film havinga thickness of 50 μm.

Example 3

A heat shrinkable polyester film having a thickness of 50 μm wasobtained in a manner similar to that of Example 1 except using apolyester obtained by mixing together 6 wt % of Polyester A, 84 wt % ofPolyester B and 10 wt % of Polyester C.

Example 4

A heat shrinkable polyester film having a thickness of 50 μm wasobtained in a manner similar to that of Example 1 except using apolyester obtained by mixing together 15 wt % of Polyester A, 75 wt % ofPolyester B and 10 wt % of Polyester E.

Example 5

A heat shrinkable polyester film having a thickness of 50 μm wasobtained in a manner similar to that of Example 1 except drawing theundrawn film in the transverse direction at the film temperature of 72°C. by a drawing factor of 5.5 (first drawing step).

Example 6

A heat shrinkable polyester film having a thickness of 50 μm wasobtained in a manner similar to that of Example 1 except drawing theundrawn film in the longitudinal direction by a drawing factor of 1.5.

Comparative Example 1

A heat shrinkable polyester film having a thickness of 50 μm wasobtained in a manner similar to that of Example 1 except that the firstdrawing film temperature was 80° C.

Comparative Example 2

A heat shrinkable polyester film-having a thickness of 50 μm wasobtained in a manner similar to that of Example 2 except that the firstdrawing film temperature was 80° C.

Comparative Example 3

A polyester obtained by mixing together 20 wt % of Polyester A, 70 wt %of polyester B and 10 wt % of polyester C was melted and extrudedthrough a T die at a temperature of 280° C., and rapidly cooled down byusing a chill roll to obtain an undrawn film.

The undrawn film was drawn with a tenter in the transverse direction bya drawing factor of 4.0 at a film temperature of 73° C., after the filmtemperature was pre-heated until reaching 85° C. under the heat transfercoefficient of 0.033 W/(m·° C.). The film was then subjected to a heattreatment at 68° C. for 10 sec to obtain a heat shrinkable polyesterfilm having a thickness of 50 μm.

Comparative Example 4

A heat shrinkable polyester film having a thickness of 50 μm wasobtained in a manner similar to that of Comparative Example 3 exceptusing a polyester obtained by mixing together 25 wt % of polyester A, 50wt % of polyester B and 25 wt % of Polyester G, and drawing the film inthe transverse direction at a film temperature of 74° C.

The evaluation results for the respective films of Examples 1-6 andComparative Examples 1-4 are shown together in Table 1 below.

TABLE 1 Drawing factor Draw- Materials Longi- Trans- ing Hot watershrinkage Final Com- Poly- Poly- Poly- Poly- Poly- Poly- Poly- tudinalverse temper- along the main shrink- pressive Thickness ester esterester ester ester ester ester direc- direc- ature shrinkage direction(%) age strength distri- A B C D E F G tion tion (° C.) 70° C. 75° C.80° C. property (mN) bution Example 1 20 70 10 — — — — 1.1 5.0 70 27 4655 ◯ 6300 ◯ Example 2 10 70 — 20 — — — 1.1 5.0 68 30 46 52 ◯ 6600 ◯Example 3 6 84 10 — — — — 1.1 5.0 71 27 47 58 ◯ 6300 ◯ Example 4 15 75 —— 10 — — 1.1 5.0 71 34 47 53 ◯ 6400 ◯ Example 5 20 70 10 — — — — 1.1 6.072 27 46 58 ◯ 6800 ◯ Example 6 20 70 10 — — — — 1.5 5.0 70 27 46 55 ◯6700 ◯ Com- 20 70 10 — — — — 1.1 5.0 80 22 40 50 Δ 5700 Δ parativeexample 1 Com- 10 70 — 20 — — — 1.1 5.0 80 25 38 48 Δ 5800 Δ parativeexample 2 Com- 20 70 10 — — — — 1.0 4.0 70 27 46 54 ◯ 5900 ◯ parativeexample 3 Com- 25 — — — — 50 25 1.0 4.0 74 28 46 55 ◯ 4950 ◯ parativeexample 4 Polyester A: TPA//EG = 100//100 (mol %) Polyester B:TPA/IPA//EG = 78/22//100 Polyester C: TPA/IPA/AA//BD = 65/10/25//100Polyester D: TPA/AA//EG = 70/30//100 Polyester E: TPA/SA//EG =53/47//100 Polyester F: TPA//EG/NPG = 100//70/30 Polyester G: TPA//BD =100//100

As is apparent from Table 1, each of the heat shrinkable polyester filmsobtained in Examples 1-6 exhibit a good final shrinkage property, asufficient compressive strength and a good thickness distribution. Theheat shrinkable polyester film of the present invention has a highquality and a high practicability, and is particularly suitable for ashrinkable label use.

On the other hand, each of the heat shrinkable films obtained inComparative Examples 1 and 2 exhibited poor final shrinkage property andpoor thickness distribution. Moreover, the heat shrinkable filmsobtained in Comparative Examples 3 and 4 exhibited poor compressivestrength. Thus, each of the heat shrinkable polyester films obtained inComparative Examples 1-4 had a poor quality and a low practicability.

According to the present invention, a heat shrinkage film with a goodfinal shrinkage property and with very few creases, uniform shrinkage,minimal distortion, and sufficient shrinkage occurring from a heatshrinkage process can be obtained. Furthermore, the obtained heatshrinkage film has a sufficient film rigidity which endures a high-speedapplying process, and a shrinkage property which shows a high shrinkagein a short time.

Therefore, the heat shrinkable polyester film of the present inventionis suitable for use in PET bottle label applications, in which ahigh-speed label application and shrinkage process is desired.

1. A heat shrinkable polyester film whose hot water shrinkage along itsmain shrinkage direction is equal to or greater than 20% when the filmis put in hot water of 70° C. for 5 sec; 35% to 55% when put in hotwater of 75° C. for 5 sec; and 50% to 60% when put in hot water of 80°C. for 5 sec, wherein the compressive strength of a label formed by thefilm satisfies a following expression:y>1.1x^(2.2) where y denotes compressive strength (mN) and x denotesfilm thickness (μm).
 2. A heat shrinkable polyester film according toclaim 1, wherein the film has a thickness distribution of 6% or less. 3.A label for a bottle which is produced by a heat shrinkable polyesterfilm according to claim
 1. 4. A heat shrinkable polyester film accordingto claim 1, wherein the compressive strength of the label formed by thefilm satisfies a following expression:y>1.1x^(2.2) where y denotes compressive strength (mN) and x denotesfilm thickness (μm).
 5. A heat shrinkable polyester film according toclaim 1, wherein the thickness of the film between 10 μm and 200 μM. 6.A heat shrinkable polyester film according to claim 5, wherein thethickness of the film is between 20 μM and 200 μM.
 7. A heat shrinkablepolyester film according to claim 1, wherein the polyester has an acidcomponent and a diol component, wherein the main acid component isterephthalic acid, mixed with 14 mol % or more of isophthalic acid andless than 5 mol % of an adipic acid, and wherein the main diol componentis an ethylene glycol, mixed with 5 mol % to 15 mol % of butanediol. 8.A heat shrinkable polyester film according to claim 7, wherein thepolyester contains less than 4 mol % diethylene glycol.
 9. A method ofproducing a heat shrinkable polyester film consisting of: a) drying apolyester material; b) melting the polyester material; c) extruding itinto a film at a temperature of 200° C. to 300° C. and rapidly coolingthe film to obtain an undrawn film; d) drawing the undrawn film at atemperature equal to or greater than Tg−5° C. and less than Tg+15° C.along a first direction by a drawing factor of 1.05 or more; e)pre-heating the film; f) drawing the film along a second direction(perpendicular to the first direction) by a drawing factor of 4.5 ormore (first drawing step); g) drawing the film further at a temperatureof 65° C. to 85° C. along the second direction by a drawing factor of1.05 or more (second drawing step), wherein Tg is the glass transitiontemperature of the film.
 10. The method of claim 9, wherein the drawingtemperature in step d) is 70° C.-90° C.
 11. The method of claim 9,wherein the drawing factor instep d) is 1.05 to 1.20.
 12. The method ofclaim 9, wherein the drawing factor in step d) is 4.7 to 5.2.
 13. Themethod of claim 9, wherein the drawing is a biaxial drawing in which thefilm is drawn in the first direction and then drawn with a tenter in thesecond direction.
 14. The method of claim 13, wherein the biaxialdrawing is a successive biaxial drawing.
 15. The method of claim 13,wherein the biaxial drawing is a simultaneous biaxial drawing.
 16. Themethod of claim 9, wherein the first direction is the longitudinaldirection (extrusion direction) and the second direction is thetransverse direction (perpendicular to the extrusion direction).
 17. Aheat shrinkable polyester film produced by the method of claim
 9. 18. Aheat shrinkable polyester film according to claim 17, wherein thethickness of the film is between 10 μM and 200 μM.
 19. A heat shrinkablepolyester film according to claim 17, wherein the thickness of the filmis between 20 μM and 200 μM.
 20. A heat shrinkable polyester filmaccording to claim 17, whose hot water shrinkage along its mainshrinkage direction is equal to or greater than 20% when the film is putin hot water of 70° C. for 5 sec; 35% to 55% when put in hot water of75° C. for 5 sec; and 50% to 60% when put in hot water of 80° C. for 5sec, wherein the compressive strength of a label formed by the filmsatisfies a following expression:y>1.1x^(2.2) where y denotes compressive strength (mN) and x denotesfilm thickness (μm).